While most current satellites use a chemical bipropellant propulsion system, a XIPS-equipped satellite instead uses the impulse generated by a thruster ejecting electrically charged particles at high velocities. XIPS requires only one propellant, xenon, and does its stationkeeping job using a fraction of that required by a chemical propellant system.
The heart of the XIPS is the ion thruster, measuring less than 10 inches across. Two other key units include a tank containing xenon gas and a power processor. Thrust is created by accelerating the positive ions through a series of gridded electrodes at one end of the thrust chamber. The electrodes, known as an ion extraction assembly, create more than 3,000 tiny beams of thrust. The beams are, to some extent, prevented from being electrically attracted back to the thruster by an external electron-emitting device called a neutralizer.
For example, Ions ejected by the Hughes-designed XIPS travel in an invisible stream at a speed of 30 kilometers per second (62,900 miles per hour), nearly 10 times that of its chemical counterpart. And, because ion thrusters operate at lower force levels, attitude disturbances during thruster operation are reduced, further simplifying the stationkeeping task.
Chemical thrusters in use today are limited by how much energy is released during the combustion process. Ion thrusters are dependent on the amount of electrical power available. More power means faster-moving ions and higher thrust. The Boeing 601HP XIPS uses 500 watts from the satellite's 8-kilowatt solar array. For the Boeing 702 model, XIPS uses 4,500 watts from the 10- to 15-kilowatt solar array. XIPS operations have no effect on broadcasting and telemetry operations.
A typical satellite will use up to four XIPS thrusters (two primary, two redundant) for stationkeeping, all connected to the same xenon supply. Each primary device will be switched on and off by a smart power unit that monitors and diagnoses operations automatically. In normal operation, each Boeing 601HP ion thruster will operate for approximately 5 hours per day. Each Boeing 702 ion thruster will operate for approximately 30 minutes per day.
An ion thruster moves ions by electrostatic repulsion. Xenon propellant enters from a nozzle. A cathode emits electrons that slam into the xenon atoms knocking loose an electron and creating positive xenon ions. Ions respond to magnetic and electric fields, and these ions are attracted to a positive grid at the back of the firing chamber. The ions are pushed by gas pressure through holes in the positive grid. Then the electric field between the positive and negative grids accelerates the ions and sprays them out the back. The grid's electric field accelerates the ions into a ghostly blue beam traveling at about 60,000 miles per hour. The electrons then are introduced into the flow to neutralize the beam. One problem with the use of an ion thruster is damage and loss of performance due to impingement of ion plumes on spacecraft.
The problem arises from the effect of Earth's magnetic field on the ions that are ejected by an ion thruster. Like any charged object moving in a magnetic field, ions experience a force that is perpendicular to their velocity and to the magnetic field:F=qv×B   (1)where boldface indicates a vector quantity, F denotes force, q is electric charge, v is velocity, and B is magnetic field. Ions moving non-perpendicular to the magnetic field follow a spiral path. Those moving perpendicular to the magnetic field travel in a circle. The size of the circle followed by a charged object is given by:R=mv/(qB)   (2)where R is the circle's radius and m is the mass of the particle. A stronger magnetic field makes the circles smaller.
Ions moving in a circle can return to strike the spacecraft from which they were ejected. At GEO altitude, the Earth's magnetic field is weak: less than 300 nanoTesla. Furthermore, thrusters used to date use xenon as propellant, so the ions are relatively massive. The result is that prior thrusters eject ions that follow very large circles: over 2000 km circumference at Isp of 6500 sec. In addition, present-day ion thrusters do not shoot ions out in a tightly collimated beam; so the ions spread out as they travel. Even when the thrust vector is perpendicular to the local magnetic field, the ions travel a large circle and the beam diverges strongly so the ion stream is greatly diminished in intensity when it returns to the spacecraft.
In future applications, ion impingement may be more significant than in GEO altitudes. An example will illustrate the problem. The Jupiter Icy Moons Orbiter (JIMO) is a $3 billion NASA program that will use about 10 metric tons of xenon during its mission. The planned initial orbit of JIMO is a circle 1000 km above the Earth. The magnetic field at that altitude is about 50,000 nanoTesla. JIMO thrusters will expel xenon ions at 6500 sec of Specific Impulse (Isp) (˜64 km/sec). These ions will travel about 10.8 km before returning to the spacecraft. Assuming the ion beam's divergence angle is 0.1 radian (full width), the beam width after one cycle is 1.08 km. Assuming the beam is mostly perpendicular to the field lines, a 10 meter wide spacecraft (reasonable for JIMO's solar arrays plus high-gain antenna) catches ˜1% of the beam.
There are several consequences. One problem is lost thrust: the ions striking the spacecraft carry momentum and strike from the front, destroying 1% of the nominal thrust. Dispelling heat is another problem: JIMO has about 100 kW of kinetic power in the ion beam, so the impinging ions add ˜1 kW of thermal load to the vehicle's radiators. This requires more radiator area, and therefore more mass. A third problem is erosion: if its ion thruster expels 1 metric ton of Xe getting from LEO to higher orbit, then JIMO catches around 10 kg of xenon ions at 64 kilometers per second. If the ion impacts cause a comparable mass of material to be sputtered off, the spacecraft's coatings could be seriously degraded. If some of the sputtered material collects on optics or other sensitive surfaces, mission performance could be compromised. A fourth problem is charge deposition: 10 kg of xenon ions carry a total charge of 7.54 million coulombs. Given the nominal performance of JIMO (eighteen months to spiral out of Earth orbit), the charging rate is 0.16 amps. This is not a lot by itself, but it is a non-trivial addition to the charging rate due to the natural environment. Conductive coatings would suffer rapid erosion due to the ion impingement, and, therefore, would need to be thicker to dissipate surface charge.
What are needed are a method and an apparatus for avoiding ion impingement on vehicles using ion thrusters.